Amplification that runs to completion
Positive feedback does the opposite of the stabilizing loop: the response increases its own trigger, so the signal grows and grows. Left alone this would be runaway and dangerous, so the body uses it only for events that should be fast, decisive, and self-terminating — they amplify until a goal is reached, then an outside event ends them.
The classic endocrine example is the LH surge that triggers ovulation. For most of the cycle, estrogen suppresses LH (ordinary negative feedback). But once estrogen rises high enough and stays there, the relationship flips: high estrogen now stimulates LH, which drives the ovary to make even more estrogen, which drives even more LH. The loop explodes upward into a surge that releases the egg — and then ovulation removes the source, ending the loop.
Other surges, and how they end
- Childbirth (parturition): the baby's head stretches the cervix, which releases oxytocin, which strengthens contractions, which push the head harder — a rising spiral that ends only when the baby is delivered and the stretch is gone.
- Milk let-down (milk let-down reflex): suckling triggers oxytocin, which ejects milk; more milk and continued suckling sustain the release until the feed is over.
In every case the pattern is the same: amplify hard, reach the goal, then a physical event (egg released, baby delivered, feed finished) removes the trigger and the loop collapses. Positive feedback is a tool for getting somewhere, not for staying put.
Feedforward: acting before the problem
Feedforward is a third pattern, and a clever one. Instead of waiting for a variable to move and then correcting it, the body acts on an early warning before the variable changes. The best example is the incretin effect: gut hormones released the moment food arrives prime the pancreas to release insulin *before* the meal's glucose has even been absorbed.